To achieve the scientific goals, we addressed the underlying fundamental scientific questions in prototypical materials.
For this, we studied the carrier thermalization, that is the timescale that optically excited carriers take to reach the thermodynamical equilibrium within the carriers (to reach a thermal distribution) in a series of different metals. Our study shows that the timescale of this process vary between metals over orders of magintude, reaching from hundreds of femtoseconds to only a few femtoseconds. Using relatively simple theoretical models, we could show that this depends on several key electronic structure properties, such as Fermi energy, plasma frequency, density of states and the carrier localization, which affects the rate of energy exchange between carriers. The latter also affects an observable spectroscopic signature of core-level transitions, as expressed through the local field effect (LFE). The results are currently undergoing peer-review.
We also studied the effect of carrier relaxation on coherent phonon motion that follows the change in energy that follows to photoexcitation. We were able to show that the relaxation of carriers and formation of a thermal carrier distribution affects the phase of the phonon oscillation. This is due to the difference in force that the carriers enact onto to lattice during the first femtoseconds following optical excitation while they occupy higher energetic states than after they form their thermal distribution. The results are currently undergoing peer-review.
To study the spin-polarization in 2D materials, we studied the possibilities to follow electron spin dynamics in semiconductors using circular polarized XUV laser pulses of attosecond duration. For this we first examined the angular-momentum selection rules that arise when two circular polarized pulses, pump and probe are used. Our results examining circular-dichroic attosecond transient absorption spectroscopy (cDATAS) in helium demonstrate that the different optical response between a co- and counter-rotating pump pulse (with respect to the probe pulse) are dependent on the magnetic spin quantum number and therefore spin-selectivity can be achieved.
We then applied the principle to a simple semiconductor, where our results indicate that the spin-polarization of holes, following optical excitation, decays much faster than shown in previous experimental results using optical methods in the visible and infra-red spectral regions. The results are currently being compared to theoretical modelling and prepared for publication.
In the final part, we were able to achieve a long standing milestone of the attosecond community, performance of an all-attosecond transient spectroscopy experiment on a solid state system. Typical experiments so far had to rely on a at least few-femtosecond duration and low-frequency (ie up to UV photon energies) laser pulse to initiate dynamics to be probed by an extreme ultra-violet pulse of attosecond duration. This limitation was mainly due to the limited conversion efficiency of the High Harmonic Generation process, which converts femtosecond duration laser pulses from commercial amplifier systems into the attosecond duration extreme ultra-violet pulses, limiting them to the linear interaction strength regime. Recent advances in the understanding the scalings of the conversion efficiency, power limitations and beamline design have enabled us to perform attosecond pulse pump and attosecond pulse probe experiments at high repetition rates in a laboratory-scale experiment. This breakthrough allowed us to then perform the all-attosecond transient reflection spectroscopy experiment on core-exciton resonances in an ionic crystal sample. The experience gathered in the first phase of the project proved invaluable in achieving this goal, both in the technical expertise as well as the scientific understanding that was gathered.
